Chapter 8 Ignition System Fundamentals 147 Copyright by Goodheart-Willcox Co., Inc. systems, the ignition control module is often installed inside of the distributor or on the outside of the distributor body. Some modules are located on the engine firewall or inner fender to remove them from engine heat. The vehicle battery provides the ignition control module with low-voltage current. Battery power is sent to the module only when the ignition switch is in the on or start position. The module passes this low-voltage current to the coil through a power transistor. The power transistor is a solid-state device that allows current to flow based on electrical inputs from other module circuitry. Current flows from the power transistor and the coil primary windings to ground. This current flow creates a magnetic field around the coil windings, Figure 8-8A. At this time, the power transistor is a conductor, allowing full current to flow in the circuit. Figure 8-7. This ignition control module is located under the coils on a distributorless ignition system. Two of the three coils are removed to show the module. The module is part of the coil- mounting bracket and is replaced as an assembly. Ignition control module Figure 8-6. This pencil coil cross section shows the windings and the magnetic core. Note that the ignition control module is not a part of the assembly. Primary connections Primary windings Secondary windings Magnetic core Secondary (high- voltage) output Insulation Figure 8-8. A—Ignition systems use a power transistor to control power flow through the coil(s). The power transistor is controlled by a low-voltage signal from the pickup coil. In this figure, the power transistor is on and current flows through the coil, building up the magnetic field. B—In this figure, the power transistor is off, stopping current flow to the coil. This causes the coil’s magnetic field to collapse and discharge through the secondary terminal. Ignition switch Current flows in curcuit Power transistor Ignition control module Other module circuitry Triggering device (pickup coil or Hall-effect switch) On Magnetic field builds up in coil To spark plugs Battery A Battery To spark plugs Ignition switch Coil magnetic field collapses, coil fires Ignition control module Power transistor Signal to module Other module circuitry Triggering device (pickup coil or Hall-effect switch) No current flow Off B